The power factor (PF) of an AC-DC LED driver should be limited within a certain range, in order to meet harmonic standards. Typical AC-DC LED drivers may be of a single-stage type or of a two-stage type. The single-stage type of AC-DC LED driver may have a large PF value by providing suitable parameters. However, control variables are selected in a limited scope, and may not be suitable for optimizing efficiency of the LED driver. An electrolytic capacitor should be included, which may however reduce the lifetime of the LED driver, and can cause flicker that may be observable by human eyes.